the pallet racks are fitted with casters
or wheels. The racks or trolleys are usually moved on rails
by a pushing mechanism, usually hydraulically powered.
Such mechanised tunnel freezers are known as pushthrough
tunnels or carrier freezers which have two tiers,
one on top of the other. These freezers are designed
primarily for packaged goods, as well as carcasses.
Advantages of mechanised freezers over batch freezers
include: improved air circulation over the product as it
moves at a steady rate through the tunnel; labour costs
are considerably decreased as pallets are not manually
placed in the freezer; and there is added flexibility of the
facility by varying the freezing time with the speed of the
ram[18]. Heat transfer coefficients in mechanised freezers
are similar to batch freezers being less than 50 W/m2 K.